Conventionally, a self-hold type thermal protector including a ceramic PTC (Positive Temperature Coefficient) element connected in parallel with a contact circuit is used as a device for preventing a temperature of an electric product from rising too high.
Such thermal protectors are intended to mainly prevent a temperature of an electric product using a commercial power supply from rising too high, and some thermal protectors control a disruption of an electric current of a voltage as high as 100 to 200V.
However, in some thermal protectors, a ceramic PTC element is used as a device for preventing a temperature from rising too high even in an area using an electric current of a low voltage such as a battery pack.
If such a thermal protector intended to prevent a temperature from rising too high is used in a circuit having a voltage equal to or lower than a commercial power supply voltage, a polymer PTC element having a low resistance is available as an embedded PTC element.
The principle of disrupting an electric current of this polymer PTC element is that a conductive path via conductive particles dispersed in a polymer is disconnected by a volume expansion caused by a thermal expansion in the vicinity of the melting point of the polymer due to an increase in a temperature, leading to a rapid increase in an internal resistance, which significantly reduces an electric current.
In the meantime, a phenomenon wherein an electric current locally gathers a hot spot can be possibly caused if a volume expansion is hindered for any reason.
FIG. 5 is a cross-sectional view of a PTC conductive polymer device disclosed by Patent Document 1. The PTC conductive polymer device has a housing composed of a case 1, and an insulative member 11 for sealing an opening of the case 1. Moreover, a first metal member 2 and a second metal member 3 are held by the housing.
For the first metal member 2 and the second metal member 3, terminal elements 21 and 31 that respectively protrude outside from the housing are formed, and holding elements 22 and 32 that are bent in an inwardly convex shape are formed within the housing.
At close to the middle of the holding elements 22 and 32, upwardly convex parts 221 and 321 are respectively formed at nearly facing positions. A PTC element 43 having layered metal electrodes 41 and 42 on both surfaces is held between the upwardly convex parts 221 and 321.
In this PTC conductive polymer device, the electrodes 41 and 42 of the PTC element 43 are pushed into a narrow space by the upwardly convex parts 221 and 321. Therefore, it is possible for the above described hot spot to occur when the PTC element 43 produces heat.
Additionally, if a current disrupt circuit implemented with a bimetal is embedded in parallel with the holding elements 22 and 32 in order to convert the structure of the PTC conductive polymer device into a self-hold type, heat produced by the PTC element 43 cannot be effectively conducted to the bimetal in a structure in which the PTC element 43 is arranged between the holding elements 22 and 32. Therefore, the structure of the PTC conductive polymer device that is illustrated in FIG. 5 and disclosed by Patent Document 1 is not applicable to a self-hold type.
A self-hold type thermal protector adopting a ceramic PTC element is well known.
FIG. 6 is a perspective top view and aside sectional view of a structure of a self-hold type thermal protector adopting a conventional ceramic PTC element. The self-hold type thermal protector 50 has a housing composed of an insulative case 51-1 and an insulative seal member 51-2 for sealing an opening of the insulative case 51-1.
Within the housing, a movable plate 53 made of a metal plate having high thermal conductivity, a bimetal 54 attached to the movable plate 53, a movable contact 55 provided at a movable side end of the movable plate 53, a first conductive member 57 having a fixed contact 56 at a position facing the movable contact 55, a ceramic PTC element 58 arranged in contact with a lower surface of a fixed side end of the movable plate 53, and a second conductive member 59 arranged in contact with an upper surface of the fixed side end of the movable plate 53 are provided.
The second conductive member 59, the fixed side end of the movable plate 53, and the ceramic PTC element 58 are aligned by a support column 52, and the second conductive member 59 and the ceramic PTC element 58 that are arranged to interpose the fixed side end of the movable plate 53 therebetween are swaged by the top and the bottom ends of the support column 52, whereby the second conductive member 59, the fixed side end of the movable plate 53, and the ceramic PTC element 58 are pressed and fixed.
Additionally, for the first conductive member 57 and the second conductive member 59, a first terminal part 57-1 and a second terminal part 59-1 that respectively protrude outside from the housing in order to connect to an external circuit are formed.
In this self-hold type thermal protector 50, the movable side end of the movable plate 53 is moved upward by the bimetal 54, which is a bimetallic element, and inversely warps with an increase in an ambient temperature. As a result, the movable contact 55 moves upward from a closed position illustrated in FIG. 6 to open a contact circuit with the fixed contact 56, whereby an electric current between the first terminal part 57-1 and the second terminal part 59-1 is disrupted.
On upper and lower surfaces of the ceramic PTC element 58, thin-layer electrodes are respectively formed. The electric current disrupted between the first terminal part 57-1 and the second terminal part 59-1 flows into the ceramic PTC element 58 via the electrodes positioned on the upper and the lower surfaces.
As a result, the ceramic PTC element 58 produces heat, and the inverted warp state of the bimetal 54, namely, the current disrupt state of the self-hold type thermal protector 50, is maintained, and at the same time, the electric current flowing into the ceramic PTC element 58 is significantly reduced by an increase in an electric resistance value with heat production.
In the meantime, in the conventional self-hold type thermal protector 50 illustrated in FIG. 6, the sides of the electrodes positioned on the upper and the lower surfaces of the ceramic PTC element 58 are respectively pressed against the fixed side end of the movable plate 53 and the first conductive member 57 by being swaged by the support column 52 in order to effectively conduct the heat produced by the ceramic PTC element 58 to the bimetal 54.
For the ceramic PTC element 58, its volume expansion by heat production is small enough to be ignorable. Accordingly, there is no possibility that the hot spot described in the PTC conductive polymer device will not occur.
However, if the resistive element (ceramic PTC element 58) is arranged in the conventional self-hold type thermal protector 50 as illustrated in FIG. 6, the sides of the electrodes positioned on the upper and the lower surfaces are respectively pressed against the fixed side end of the movable plate 53 and the first conductive member 57 as described above, and the upper and the lower surfaces, which have the widest areas of the plate, are strongly pushed upward and downward.
Accordingly, if the polymer PTC element is used as a resistive element having a low resistance in a structure similar to that of FIG. 6, the polymer PTC element is strongly pushed upward and downward as described above. Therefore, the degree of freedom of the volume expansion caused by the thermal expansion of the polymer PTC element at the time of heat production is hindered, leading to an inevitable occurrence of the above described hot spot.    Patent Document 1: Japanese National Publication of International Patent Application No. 2000-505594